8 DERIVATION OF ORGANIC BEINGS. 



acquire their full or sexual development, not as free worms, 

 but within the body of another animal, and of a species 

 distinct from that in which they had passed the early stage 

 of their existence.* 



In illustration, Professor Owen gives a summary of the 

 reproduction of the Distomata, which we shall presently 

 have occasion to consider more at length. 



He proceeds : " The sum of the recent researches on 

 the generation of the Entozoa, teaches that, to the success 

 in life of the majority of these internal parasites, two diffe- 

 rent species of much higher organized animals are subser- 

 vient ; and that these two species stand in the relation of 

 prey and devourer. The habits of the prey favour the ac- 

 cidental introduction as when a slug crawls over the 

 droppings of a thrush of the eggs of the bird's intestinal 

 parasite. These are hatched in the slug. The slug, in its 

 turn, is devoured by the thrush, but the parasitic passen- 

 gers are not digested only the coach is dissolved, and the 

 larvae, thus set free, find in the warm intestines of the bird 

 the appropriate conditions for their metamorphosis and full 

 development. In like manner, the Rhynchobothria of a 

 cuttle fish are the larvae of the Tetrarhynchm or four-ten- 

 tacled tape worm of a dog-fish. The encysted sexless 

 Tricenophorus of the liver of the char becomes the free and 

 perfect Triwnophorus of the gut of the pike. The Ligula 

 of a herring becomes a Tcenia only when introduced into 

 the interior of a cormorant. The bladder-worm (Cysticercus 

 fasciolaris ) of the mouse's liver becomes the tape- worm 

 (Tcenia crassicollis) of the cat. The Cysticercus pisiformis 

 of the liver of the hare becomes the Tcvnia serrata of the 

 dog and fox. Dr. Kuchenmeister of Zittau first proved, ex- 

 perimentally, by feeding animals with Cysticerci (Hydatids 

 of the flesh and glands of herbivorous animals) that they 



* Address to Brit. Assoc. at Leeds, 1858. P. 23. 



